1. Field of the Invention
This invention relates to sensing diaphragm differential pressure transmitters of the type used to generate a signal, electrical or pneumatic, related to the differential pressure between a high pressure line and a low pressure line each containing a liquid or gaseous process fluid. A typical example of the use of such a device might be a flow measurement installation in which a flow restriction, such as an orifice plate, is positioned in a pipe containing a flowing process fluid. A high pressure line is connected to a high pressure tap immediately upstream of the orifice plate and a low pressure line is likewise connected to a low pressure tap immediately downstream of the orifice plate. The pressure differential between these pressure lines is known to be indicative of the flow rate of the process fluid. The pressures in these lines are caused to act against opposite faces of a sensing diaphragm having a relatively linear spring rate, the periphery of which is secured to the transmitter body to prevent motion. Motion of the central portion of the sensing diaphragm is related to the differential pressure and therefore to the flow rate. A conventional position transducer may then be used to generate a signal indicative of the flow rate.
The accuracy of such transmitters depends in part upon the linearity of the spring-diaphragm assembly. A spring member, having one end affixed to the transmitter body, is typically positioned adjacent the low pressure face of the diaphragm to resist motion of the central portion thereof so that the spring rates of the sensing diaphragm and spring are combined.
Such transmitters are occasionally subject to differential pressures of magnitudes beyond the range of their designed capabilities, usually during installation or repair. This problem is commonly referred to as overranging and, although infrequent, is quite serious because the resulting distortion of the sensing diaphragm often renders the transmitter unpredictably nonlinear in its response to differential pressures and therefore unsuitable for use.
Overranging problems occur in two modes: high side overrange and low side overrange. The present invention relates to the construction and design of such differential pressure transmitters rendering them less susceptible to damage from low side overranging.
Negative differential pressure occurs when the pressure in the low pressure line exceeds the pressure in the high pressure line. Low side overrange occurs when the negative differential pressure exceeds the design limit of the device. This condition can be caused by removal of the high pressure line from the transmitter without removal of the low pressure side, accidental application of an overrange pressure to the low pressure side of the transmitter or by other means.
2. Description of the Prior Art
Differential pressure transmitters of the type to which the instant invention is related are often utilized with either corrosive or contaminated process fluids. It is customary to protect the sensing diaphragm, as well as the process fluid, by use of a fluid filled isolation diaphragm system as follows. The sensing diaphragm is positioned within a chamber filled with a relatively incompressible fill fluid and sealed at each end by isolation diaphragms whose spring rates are designed to be insignificant relative to the effective spring rate of the sensing diaphragm and spring assembly. The pressure from the high pressure process fluid line is applied to one of the isolation diaphragms and the pressure from the low pressure line to the other. In this way the process fluid and sensing diaphragm are isolated from each other without adversely affecting the operation of the transmitter.
One example of the prior art construction of such devices is shown in U.S. Pat. No. 3,277,719 issued on Oct. 11, 1966, which shows a single sensing diaphragm differential pressure transmitter having a fluid filled isolation diaphragm system in which adjustable stops are used to mechanically limit travel of the center portion or hub of the sensing diaphragm under conditions of low pressure overrange. This hub is mechanically secured to the free end of the spring.
The stops are provided for the protection of the spring which supports the sensing diaphragm but do not prevent the fill fluid, under low side overrange conditions, from flexing the sensing diaphragm beyond its elastic limit thereby affecting its linearity. This prior art design relies on the "bottoming out" of the appropriate diaphragm against its backup surface to minimize the damage as described above but this is usually not adequate under high temperature conditions which cause the fill fluid to expand. For clarity, FIG. 3 herein shows a basic transmitter structure, not unlike that of the prior art, including the adjustable stops in order to show the low side overrange problem associated therewith.
The position transducer, whose signal is indicative of the differential pressure applied to the transmitter acts upon a structural member affixed to the free end of the spring.